Control system



N. H. WILLBYV 2,000,728

C ONTROL SYSTEM May 7, 1935.

Filed June 21, 19255 WITNESSES:

INVENTOR 2 Y Nor/77cm H Wz'ZZbg.

TORN Patented May 7, 1935 PATENT "OFFICE 2,000,123 coNraoL srs'rarr Norman E. .Wlllby, Wilkinsburg, Pa.,-asslgnor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa, a corporation of Pennsylvania Application June 21, 1933, Serial No. 676,846

8 Claims. (01. 172 -179) My invention relates, generally, to control systems and, more particularly, to systems for controlling the operation of the propelling'motors of electric vehicles.

7 An object of my invention, generally stated, is to provide a control system which shall be simple and I emcient in operation and which may be economically manufactured and installed. v v

A more specific object of my inventionis to provide for simultaneously varying the speed and the tractive eflort or torque of the propelling motors of an electric vehicle.

Another object of my invention is to provide an automatic accelerating system in which the speedoi' the motors may be reduced without first disconnecting the motors fromthe power source.

A further object of my invention is to provide a limit relay which will so control an automatic accelerating system that the motors will produce a. predetermined tractive effort.

A .still further object of my invention isto prevent the jolting of the passengers on a car when power is first applied to the propelling motors, thereby increasing the "comfort of the passengers.

Other objects of my invention will be described fully hereinafter, or will be apparent to those skilled in the art. a

'The invention, accordingly, is disclosed in the embodiment thereof shown in the accompanying drawing, and comprises the features of construction, combination of elements and arrangement of parts which will be exemplified in the construction'hereinafterset forth and the scope of the application of which will be lndicatedin the claims. l I

For a. .fuller understanding of the nature and objects of the invention, reference maybe had to the following detailed description, taken in conjunction with the accompanying drawing, in which Figure 1' is a diagrammatic view showing the apparatus and the circuits of a motor control system embodying my invention, and

Fig. .2 is a chart showing the sequence of operation of a part of the apparatus shown in Fig. l. a 7

Referring to the drawing, the system shown comprises motors l and H which are 01' a type suitable .for propelling electric vehicles. Each of the motors are provided with two series fleld windings, the motor "I having field wind-' ings t2 andl3,:and themotorllhavingfleld windings l4 and IS. A line switch It is provided for connecting the motors l0 and H to a power conductor H, which may be connected to a trolley conductor 18 by means of a trolley Hi. The trolley conductor l8 may be energized from any suitable source of power, as a'generating station (not shown).

The motors l0 and II are connected in seriescircuit relation when it is desired to start the vehicle. 'After'it has been accelerated to a predetermined speed, the motors are connected in parallel-circuit relation in order that the speed of the vehicle may be increased further. Electrically-operated switches 2!, 22, 23, 24 and 25 are provided for establishing the necessary circuitsto connect the motors in either series or parallel circuit relation.

In accordance with the usual practice, resistors '26, 21 and 28 are provided for controlling the motor current and the acceleration of the motors. Accelerating switches 3|, 32 and 33 are disposedto shunt the resistors 26, 2'! and 28 to accelerate the motors in a manner well known in the art.

With a view to varying the speed at which the motors l0 and H operate, provision is made for shunting the field windings l3 and I through resistors 34 and 35, respectively, thereby controlling the field strength of the motors and, consequently, their speed. Electricallyoperated switches 36 and 3'! are disposed to shunt the fieldwlndings l3 and I5, respectively. Provision is also made for disconnecting the field windings l3 and [fr-from the motor circuits by means of switches 38 and 39 in order that the speed of the motors may be varied were. greater range. In this manner, the field strength of the motors l0 and H may be so controlled that the motors will operate at several =difierent5 speeds corresponding to full field, shunted field and tapped field, thereby providing a wide range of speed control.

In order that the acceleration of the motors may be automatically controlled, a currentlimit'relay' ll is connected in the motor circuit as shown. The relay 4| is provided with a series coil which is divided into two sections 42 and 43 which are connected in series with the field winding l2 and I3, respectively, of the motor i0. The-coils 42 and 43 are,-therefore, energized by the current flowing through the field windings l2 and i3, respectively, and the relay M is actuated by a force proportional to the fleld'strengthof the motorl'll. Since the torque or tractive efiort of a motor is directly propor- Rfiiiilllib tional to the armature current and the field strength, the relay AI will pemit sufficient current to flow through the motors to maintain their tractive efiort at a predetermined value.

With a view to permitting the rate of acceleration of the motors to be varied to suit operating conditions, the limit relay 4| is provided with three loading coils 44, 45 and 46, which are disposed to oppose the actuating coils 42 and 43. When the loading coils are energized, the amount or" current necessary in the actuating coils 42 and 43 to open the contact members of the relay increased, thereby increasing the amount of current permitted to fiow through themotors I8 and H during each step of acceleration and, consequently, increasing the rate of acceleration of the motors. A resistor 41 is provided to control the current in the loading coils 44, 45 and 46.

The limit relay 4! cooperates with a sequence switch 5i to automatically control the operation of the transfer, accelerating and field shunting switches, previously described. The sequence switch 5! is automatically advanced step-by-step to control the operation of the foregoing switches upon the actuation of a master controller 52 to set it in operation. As illustrated, the sequence switch comprises a plurality of contact fingers disposed to engage contact se ments mounted upon a drum which is actuated by an air engine 53.

The operation of the air engine 53 is controlled by magnet valves 54 and 55 which are actuated by magnet coils 56 and 5'1, respectively. The air engine 53 is so constructed that the drum of the sequence switch 5| is advanced from position a toward position "112. when both of the magnet coils 56 and 51 are energized. When both of the coils 56 and 57 are deenergized, the drum is actuated toward position a. The drum may be stopped and retained in any intermediate position by energizing the coil 56 and deenergizing the coil 5'. simultaneously.

A lost motion device 58 is incorporated in the controller 52 in order that the sequence switch may be readily stopped at any time. The lost motion device is similar in construction to, and functions in the same manner as, the device described in my copending application Serial No. 676,845, filed June 21, 1933 and assigned to the Westinghouse Electric & Manufacturing Company. A slight backward movement of the controller handle interrupts the circuit through the magnet coil 5'! and stops the sequence switch.

In order that the speed of the motors may be reduced without first disconnecting them from the power source, as has been necessary in previously known automatic accelerating systems, the master controller 52 and the sequence switch 5i are so constructed that the drum of the sequence switch may be actuated to any one of several positions corresponding to certain positions of the master controller by actuating the controller to the desired position. For example, if the master controller and the sequence switch are on positions g and m, respectively, in which case the motors will be operating at maximum speed, and it is desired to operate at a lower speed, the master controller may be actuated to any intermediate position, as, for instance, position and the sequence drum will be immediately returned to position g, thereby causing the motors to operate at a speed corresponding to position 0 of the controller.

With a view to preventing the uncomfortable jolt experienced by the passengers when power is first applied to the motors, provision is made for reducing the torque of the motors at the instant of starting and gradually increasing the motor torque to its maximum value. In the present embodiment of the invention, the field windings I3 and I of the motors are shunted when the motors are first connected to the power source, thereby reducing the field strength and the torque of the motors. Full field strength'is applied shortly after the motors are connected to thepower source. In this manner the torque of the motors is increased gradually and the vehicle is started without jolting the passengers.

In order that the speed and the tractive effort of the motors may be varied simultaneously by the operator, contact segments 53 and 60 are so disposed on the master controller 52 that the energization of the loading coils 44, 45 and 46, on the limit relay 4|, and also the energization of the field windings I3 and I5 may be controlled by operating the controller 52. As explained hereinbefore, the tractive effort of the motors may be varied by controlling the energization of the loading coils on the limit relay, which is accomplished by means of the contact segment 59 on the master controller. The operation of the switches 36 and 38, and 31 and 39 which govcm the energization of the field windings l3 and I5, respectively, is controlled by the sequence switch 5|. However, the position to which the sequence switch may advance is controlled by the position on which the master comptroller is set. Thus, if the controller is set on position e, the sequence switch will be stopped on position k and the switches 36 and 31 will not be closed to shunt the field windings I3 and I5. In this manner, the speed at which the motors I 0 and II operate may be controlled.

In order that the functioning of the foregoing apparatus may be more clearly understood, the operation of the system will now be described. Assuming that it is desired to connect the motors I0 and II to the power conductor IT to start the vehicle, the master controller 52 may be actuated to position a, thereby establishing an energizing circuit for the actuating coil of the line switch I6. This circuit may be traced from a conductor 6i, which is connected to the trolley I9, through contact fingers 62 and 63, bridged by the contact segment 60, conductor 64 and the actuating coil of the switch I6 to a grounded conductor 65.

At this time, a circuit is also established through the actuating coil of the switch 23, which may be traced from a contact finger 66 on the controller 52, through conductor 61, an interlock 68 on the switch 25, conductors 69 and H, and the actuating coil of the switch 23 to the grounded conductor 65.

As explained hereinbefore, provision is made for shunting the field windings I3 and I5 of the motors Ill and II when power is first applied to the motors in order to reduce the initial torque of the motor. Accordingly, energizing circuits are established at this time for the actuating coils of the switches 36 and 31, which are disposed to shunt the field windings I3 and I5, respectively, through the resistors 34 and 35. The circuit through the actuating coil of the switch 36 may be traced from the conductor SI through conductor 12, an interlock 13 on the switch I6, conductors 14 and 15, an interlock 16 on the TIN emorse switch 33, conductor 11, contact fingers 13 and 1 8, bridgedby acontact segment 38 on the sequence switch 5|, conductors 8| and 82, and the actuating coil of the switch 38 to the grounded conductor 85. The actuating coil 01' the switch 31 is energized simultaneously with thecoil'of the switch 88 through a circuit which extends from theconductor 3| through the coil of the switch 31,. conductor 83, an interlock 34 on the switch 2| and conductor 85 to the grounded conductor 85. I 4

The motors"! and II are now connected in series-circuit relation and are connected to the power conductor I1. tracedirom the conductor I1 through the contact members 88 oi the switch I8, conductor '81, thearmaturewinding of the motor III, the field winding I2 of the motor III, the series coil=42 oi the limit relay'4I, conductors" and 88, the resister 34, conductor 9|, the contact'memhers'n of the switch, conductors 83 and 34, the resisters 21 and 28, conductor 35, the contact members 88 of the switch :23, conductor 31, the resistor 28, conductorj98 oi the contact members 38 of the switch 31, conductor I8I, the resistor 35, conductors I02 and I83, the field winding I4, the armature winding of'the motor II and con- :uctors I04 and I05 to the grounded conductor 5. I a I Upon the closing of the switch38, acircuit is establishedfor the actuating coil'ot the switch 38, thereby closing this switch to connect the field winding I3 in the motorcircuit to increase the torque developed by motor III. This circuit may be traced from the previously energized conductor 14 through an interlock I88 on the switch 22, conductor I81, an interlock I on the switch 88, conductor I89 and, the actuating coil of the switch 38 to thegrounded conductor 85.

When the switch-38 is closed, an energizing circuit is established for the actuating coil of the switch 38 to close this switch and connect the fieldwinding I5 in the motor circuit. This circult may be traced from the previously energized conductor I08 through an interlock III on the switch 38, conductor N2, the actuating coil of the switch 39, conductor 83, the interlock 8,4 and conductor 85 to the grounded conductor 85., It will thus be seen that the field windings I3 and II are energizedsuccessively,thereby gradually increasing the tractive eflort of the motors I8 and Il,which will cause the vehicle to be started smoothly instead of with a sudden jerk.

When the switch 39 is closed, the energizing circuit for the actuating coils of the switches 38 and 31 is interrupted by the opening of the interlock 18 on the switch 39, therebycausing the switches 38 and 81 to be opened, which interrupts the shunt connections around the field windings I3 and I5 of'the motors I8 and II. In this manner full field excitation is applied to the motors, which causes them to develop their maximum tractive eilort.

The motors I and II may be accelerated by actuating the controller 52 to position b, thereby establishing an actuating circuit for the magnet coil 51, which will cause the sequence switch to be operated by the air engine 53.

As explained hereinbefore, the drum of the sequence switch 5| is advanced when the magnet coils 58 and 51 are both energized. The magnet coil 58 was energized when the controller 52 was actuated to position a, through a circuit which extends from a contact finger I The motor circuit may be and H8, bridged by the contact segment 'II1, conductor II8, the magnet coil '58 and conductor II9 to the'grounded conductor 65. The circuit through the magnet coil 51 may be traced from a contact finger I2I through conduc tor I22, contact fingers I23 and I24, bridged by contact segment I25, conductors I26 and I21, contact members I28 and I29 of the lost motion device 58, previously described, conductor'il3l, the contact members of the limit relay 4I, conductors I32 and I33, the magnet coil 51 and conductor II9 to the grounded conductor 85.

When the sequence switch 5| advances to po-v closing this switch to shunt the resistor 21. This circuit may be traced from the conductor 8| through contactfingers I34 and I35, bridged by a contact segment I36, conductor I31 and the actuating coil of the switch 33 to the grounded conductor85. At this time, a contact finger I38 also engages the contact segmentI36 to energize aconductor I39 which is connected to the conductor I09, thereby maintaining the energization of the actuating coils of the switches 38 and '39.

As'the sequence switch passes through position d toposition e, an energizing circuit is established for the actuating coil of the switch 32. This circuit maybe traced from contact finger |4I through conductorl42 and the actuating coil of 'the switch 32 to the grounded conductor '85. When the switch 32 is closed, theresistor 28 is shunted from the motor circuit.

When the sequence switch advances to position I, the switch 3| is closed to shunt the resistor 28 from the motor circuit. The circuit through the actuating coil of the switch 3| extends from a contact finger I 43 through conduct-or I44, the actuating coil of the switch 3|, conductor 83, the interlock 84 and conductor 85 to the grounded conductor 85.

It will thus be seen that the series resistance has all been shunted from the motor circuit. However, as described hereinbefore, the speed of the motors may be increased by shunting the-field windings "I3 and I5 by closing the switches 38and 31. 'When the sequence drum advances to position g, the conductorlll is energized to'cause the switches 38 and 31 to be closed, as previously described.

The speed of the motors III and II may be still furtherincreased by opening the switches 38 and-39 which disconnects the field windings I3 and I5 from the motor circuit, thereby further weakening the field strength of the motors and increasing their speed. The sequence switch 5| was advanced to position "'g by actuating the controller 52 to position 0", thereby energizing the contact finger I45, conductor I48 and the contact finger I41 to maintain the energization of the magnet coils 58 and 51 through their respective circuits previously described. Likewise, the sequence switch may be advanced to position h by actuating the controller 52 to position d, thereby energizing the contact finger I48, conductor I48 and the finger I50.

When the sequence switch advances to posi tion h, the energizing circuit for the actuating coils of the switches 38 and 39 is interrupted, thereby disconnecting the field windings I3 and I5 from the motor circuit. At this time, energizing circuits are established for the actuatin coil of the switches 22 and 2l,;thereby maintaining the connections through the motors i3 and II. The energizing circuit through the actuating coil of the switch 22 may be traced from the contact finger II through conductor i522 and the coil of the switch 22 to ded conductor 65. The coil of the energized through conductor I53,

"rested to conductor I52.

The main motor circuit at this time may be traced conductor 81 through the armature of motor Iii, field winding I2, the coil 42 of the liini relay 4|, conductor 88, the contact members 554 of the switch 24, conductors I55 and the contact members 96 of the switch. conductors 81, I56 and I51, the contact members 558 of the switch 22, the field winding 53, the armature of the motor II and conductors i84- and I05 to the grounded conductor As previously explained, the operation of the sequence switch 5| during the accelerating period is controlled by the limit relay 4|. It will be readily understood that the current in the motor circuit increases when resistance is shunted from the circuit by the closing of the accelerating switches in the manner previously described. When the motor current increases above a predetermined value, the contact members of the relay 4! are opened by the force exerted by the series coil 42 and also the series coil 43 when the field winding I3 is connected in'the motor circuit. The opening of the contact members of the relay 4| interrupts the energizing circuit for the magnet coil 51 thereby stopping the sequence drum. As the motors accelerate in speed their counter electro-motive force increases and the motor current is reduced, thereby permitting the contact members of the relay 4% to close, which permits the sequence switch 5! to be advanced another step. In this manner, the sequence switch is permitted to advance step-by-step under the control of the limit relay 4| to accelerate the motors.

if it is desired to further increase the speed of the motors It and II by connecting them in parallel-circuit relation, the controller 52 may be advanced to position 2, in which position a contact finger 65 engages the contact segment to to eneig he a conductor I62, a contact finger I63 and the contact segment I25 of the sequence switch 5|, thereby keeping the conductor I26 energized and maintaining the circuit previously traced through the magnet coil 51. The sequence switch will, therefore, advance through the transition step to position 7', during which time the switches 23 and 33 are permitted to open and the switches 2| and 25 are closed to connect the motors III and- II in parallel circuit relation. The energizing circuit for the actuating coil of the switch 2| may be traced from a contact finger I64, which engages the contact segment I36, through conductor I65 and the coil of the switch 2| to the grounded conduct r $5. The circuit for the coil of the switch 25 extends from the conductor I65 through conductor I66 and the coil of the switch 25 to the grounded conductor 65.

The motors are now connected in parallel circuit relation, the circuit through the motor III extending from the conductor 81 through the armature of the motor I8, the field winding I2, the coil on the limit relay 4|, the conductor 88, contact members I54 of the switch 24, conductors I55 and 95, the resistors 28 and 21, conductor 94, contact members I61 of the switch 25 and conductor I05 to the grounded conductor 65. The circuit for the motor II may be traced from the conductor 81 through the contact members I68 of the switch 2 I, conductor I69, the resistor 26, conductors I56 and I51, the contact members I58 of the switch 22, the field winding I4, the armature winding of the motor II and conductors I04 and I05 to the grounded conductor 65.

The sequence switch will continue to advance to position is since the drum of the switch is constructed in a manner to maintain the energizing circuit for the magnet coil 51. When the sequence switch is advanced to position k, the switches 38 and 39 are closed to connect the field windings I3 and I5 in the motor circuit, as previously described.

The speed of the motors may be increased by actuating the controller 52 to position I to energize a contact finger I1I, a conductor I12 and a contact finger I13 on the sequence switch, thereby permitting the sequence switch to ad- Vance to position Z. When the sequence switch is on position I, the switches 36 and 31 are closed to shunt the field windings I3 and I5, as previously described.

\The sequence switch may be advanced to position m by actuating the controller 52 to position g to energize a contact finger I 14 to which the conductor I26 is connected.

When the sequence switch advances to position .m, the switches 3| and 32 are closed to shunt the resistors 26, 21 and 28 from the motor circuits, as previously described. At this time, the switches 38 and 39 are opened, thereby disconnecting the field windings I3 and I5 from the motor circuit which permits the motors to operate at their maximum speed.

As described hereinbefore, the limit relay 4| which controls the operation of the sequence switch 5|, thereby controlling the rate of acceleration of the motors, is provided with the loading coils 44, 45 and 46 which are disposed to oppose the actuating coils 42 and 43. The loading coil 44 is energized through a circuit which extends from a contact finger I15, which engages the contact segment I25, through conductors I15 and I11, the coil 44, conductor I18, the resistor 41 and conductor I19 to the grounded conductor 65.

When the master controller 52 is actuated to position 0, the contact segment 59 bridges contact fingers |8| and I82, thereby energizing the loading coil 45 through a circuit which extends from the conductor I11 through the coil 45, conductor I83, the contact fingers I8I and I82, conductor I18, resistor 41 and the conductor I19 to the grounded conductor 65. When the controller is actuated to position 6, the loading coil 46 is energized through a circuit which may be traced from the conductor I16 through the coil 46, conductor I84, the contact fingers I85 and I82, conductor I18, resistor 41 and conductor I19 to the grounded conductor 65. The three coils 44, 45 and 46 are thereby energized to oppose the actuating coils 42 and 43, which increases the amount of current required to opcrate the relay 4| and causes the motors I0 and II to be accelerated at a higher rate.

When the controller 52 is actuated to position 9, the rate of acceleration is still further increased. The resistor 41 is shunted from the circuit through the coils 44, 45 and 46 when the controller is on position g", as a contact finger I88 engages thecontoet segment and connects the coils 44-, ll and'fl-dlrectly tothe groundedconductor 65 through a conductor I81; In this manner, the current flowing thrmigl'i the loading coils M, 49' and 4% is increased, thereby permitting a higher amount of current renew through motors I5 and H: p

In this manner, the operator of the vehicle may vary both the rate of acceleration of the motors and the speed at which the motors operate by means of the controller 52. The highest rate of acceleration and also the maximum speed of the motors is obtained by actuating the controller to position g". Lower rates of acceleration and lower operating speeds may be obtained by retaining the controller drum on some one of the intermediate positions.

In order to prevent the sequence switch 5| from stopping between positions when the circuit through the contact members oi the relay M is interrupted, as previously described, a plurality of contact segments I" on the sequence switch 5| are disposed to be engaged by a contact finger I89 while the sequence switch is being actuated from one position to the next, thereby maintaining the energizing circuit through the magnet coil 51. In this manner, the sequence switch is prevented from stopping between operating positions.

In accordance with the usual practice, the motors l and II may be disconnected from the power source to stop the vehicle by returning the master control 52 to the ofl' position, which permits the line switch'lfito be opened to disconnect the motors from the power source. When the master controller is actuated to the ofi' position, the sequence switch BI is returned to position a, thereby permitting all of the switches controlled by it to be actuated to the deenergized position.

I do not desire to be restricted to the specific embodiment or arrangement of parts herein shown and described, since it is evident that they may be changed and modified without departing from the spirit and scope of my invention as defined in the appended claims.

I claim as my invention: o

1. In a motor control system, in combination, a motor having a normal series field winding and an additional series field winding, a source of power for the motor, switching means for controlling the energization of the additional field winding to vary the motor'speed, means for automatically controlling the acceleration of the motor, a relay for controlling the .tractive effort oi the motor, means associated with said relay for varying the tractive effort of the motor, means associated with the acceleration-controlling means for controlling the operation of said switching means, and a master controller for governing the operation of the acceleration-controlling means and the means for varying the tractive eiIort, whereby the tractive effort and the speed of the motor may be varied simultaneously.

2. In a motor control system, in combination, a motor having a normal series field winding and an additional series field winding, a source of power for the motor, switching means for controlling the energization of the additional field winding to vary the motor speed, a sequence switch for automatically controlling the acceleration of the motor, means associated with the sequence switch for controlling the operation'o! saidswitching means, a master controllet, a limitrelay-responsive to the armature cur-- rent and the-field strength of the motor and cooperating with the'master controller to control the operation of the sequence switch, means associatedwith the-limit relay for varying the tractive'efi'ort of the-motor, and meansassociated with the master controller for governing the operation of the relay, whereby the tractive efiort and the speed of the motor may be varied simultaneously.

3. In a motor control system, in combination, a motor having a field winding, a source of power for the motor, means for varying the current in the field winding of the motor to vary the motor speed, means for automatically controlling the acceleration of the motor, a relay having a coil energized by the armature current of the motor for controlling the operation of the acceleration-controlling means, means on said relay responsive to the field strength of the motor for causing the tractive effort of the motor to be maintained at a predetermined value, and a master controller for governing the operation of said relay.

4. In a motor control system, in combination, a motor having a field winding, a source of power for the motor, means for varying the current in the field winding of the motor to vary the motor speed, means for automatically controlling the acceleration of the motor, a relay having an actuating coil connected in the field winding circuit for controlling the operation of the acceleration-controlling means, means on said relay disposed to oppose the actuating coil, and a master controller for controlling the energization of the last-named means.

5. In a motor control system, in combination, a motor having a normal series field winding and an additional series field winding, a source 01' power for the motor, switching means for controlling the energization of the additional field winding to vary the motor speed, means for automatically controlling the acceleration of the motor, and a relay for controlling the operation of the acceleration-controlling means, said relay having an actuating coil connected in the normal field winding circuit and another coil connected in the additional field winding circuit, whereby the tractive effort of motor may be maintained at a predetermined value.

6. In a motor control system, in combination, a motor having a series field winding, a source of power for the motor, means for connecting the motor to the power source, means for controlling the energization of the field winding of the motor, and means for automatically reducing the field strength of the motor when power is first applied to the motor and for increasing the field strength within a predetermined time interval after the motor is connected to the power source.

7. In a motor control system, in combination, a motor having a series field winding, a source 01' power for the motor, means for connecting the motor to the'power source, switching means for controlling the energization of the field winding of the motor, and interlocking means associated with said switching means for automatically reducing the field strength of the motor and for gradually increasing the field strength after the motor is connected to the power source. I

8. In a motor control system, in combination,

a motor having a normal series field winding and an additional series field winding, a source of power for the motor, means for connecting the motor to the power source, switching means for controlling the energization of the additional field winding to vary the field strength of the motor, and interlocking means associated with the switching means for controlling the operation of said switching means to cause the field strength of the motor to be reduced when power is first applied to the motor and to be gradually increased after the motor is connected to the power source.

NORMAN H. WILLBY. 

